Optical modulators are needed for external modulation of light in the medium and long haul telecommunication markets. Dense Wavelength Division Multiplexing (DWDM) allows for more than 40 times multiplication of the system's signal carrying capacity. Reduction of cost per DWDM channel provides a competitive edge for telecommunication equipment manufacturers. Each channel requires a modulator and suitable electrical RF driver. Modulators have been developed that require switching signals of only 2-3V, enabling the use of low cost electrical drivers as a source of input modulation signals. These sources are also characterized by low power dissipation.
Lithium niobate (LiNbO3) and semiconductor versions of low switching voltage modulators are available. Lithium niobate devices use the electro-optic effect to generate phase modulation inside a Mach-Zehnder Interferometer (MZI), while semiconductor devices either use either phase modulation inside of an MZI, or amplitude modulation directly via the Electroabsorption (EA) effect. The properties of the Multiple Quantum Well (MQW) structures within the semiconductor devices have an inherent wavelength dependence that is much larger than that observed with lithium niobate. The tight process control needed to make the semiconductor devices suitable for DWDM applications is difficult to achieve, thereby making them either unavailable or unsuitable for many applications of medium distance and long haul DWDM. Low switching voltage lithium niobate devices are inherently broadband vs. wavelength, though, tend to have higher cost when compared to semiconductor modulators, due to the smaller number of devices per wafer.
U.S. Pat. No. 5,303,079 discloses a device in which external modulation is accomplished in a dual waveguide device wherein substantially identical input optical beams are supplied to the waveguides and wherein each waveguide through its electrode is subject to individual, mutually exclusive control. Modulation signals are applied to each waveguide via its separate electrode. Control signals are applied to each waveguide for adjusting the modulation chirp parameter to a desired fixed, non-zero value. Modulated lightwave signals emerging from the waveguides are combined to form a single output signal suitable for transmission over an optical fiber. However, the '079 device, initially intended for producing controlled chirp, was made in Z-cut LiNbO3, and was found to have efficiency adequate for use with low cost electrical drivers. In addition, its properties are inherently wavelength independent, therefore, it is more suitable for use in DWDM applications. However, the '079 device requires buffer and charge bleed-off layers, which increase the cost of manufacture.
A traveling wave optical modulator on X-cut lithium niobate is disclosed in U.S. Pat. No. 5,138,480. The impedance of a traveling wave optical modulator may be increased to a desired input impedance without adversely affecting the drive voltage or velocity matching of the modulator. This is accomplished in the '480 device by reducing the width of the ground electrodes to not more than 3 times the width of the hot electrode.
Optical communication methods and apparatus are disclosed in U.S. Pat. No. 5,101,450 for transmitting two or more optical signals with different optical carrier frequencies on a single optical fiber with high spectral efficiency. Each optical carrier is modulated with multiple modulated subcarriers. An optical phase modulator provides cancellation of second order intermodulation products in each optical signal, thereby permitting the optical carrier frequencies to be spaced by 2f.sub.max1 where f.sub.max is the maximum modulation frequency. In another embodiment, a single sideband optical phase modulator provides cancellation of second order intermodulation products and one signal sideband, thereby permitting the optical carrier frequencies to be spaced by f.sub.max.
The prior art includes devices that use an Electro-Absorption (EA) or Mach-Zehnder Interferometer (MZI) optical modulator, fabricated on a semi-conducting substrate like InP. The drive voltages of these semi-conductor devices are compatible with lower cost electrical drivers, which have output voltages of approximately 2 volts. However, the EA devices suffer from chirp induced in the optical wavelength during the transition from the ON to OFF state. The properties of both EA and MZI semiconductor devices are also inherently wavelength dependent, and therefore both are difficult to manufacture for applications, such as Dense Wavelength Division Multiplexing (DWDM), where the wavelength of operation must be tightly controlled.